1. Field of the Invention
The present invention relates to a sound carrier detecting circuit for use in a television or a video tape recorder, in which, during the sound carrier detection, a carder frequency of sound signals is corrected by utilizing pulse width modulation signals output from a microprocessor, so as to eliminate noise.
The following disclosure is based on Korean Patent Application 93-16138, filed Aug. 20, 1993, the disclosure of which is hereby incorporated into the present application by reference.
2. Description of the Background Art
Generally in a television or in a video tape recorder, there is installed a receiving device having a tuner for receiving signals output from a transmitting end. When a very high frequency (VHF) channel or an ultra-high frequency (UHF) channel is selected, the tuner converts signals from the selected channel into an intermediate frequency which is easy to demodulate. Then the output signals of the tuner are demodulated, and the demodulated signals are detected as video signals and sound signals and are then supplied to a video signal processing part and to a sound signal processing part, respectively.
For the conventional circuit which carries out the channel selection, the demodulation and the detection as mentioned above will be described as to its operations in detail, referring to FIG. 1.
FIG. 1 is a block diagram showing the constitution of a conventional receiving device.
The signals which are transmitted from a television broadcasting station or from a transmitting station are received through an antenna 2 to a tuner 10, and the received signals are subjected to a channel selection. The channel-selected signals are mixed with oscillating signals to be generated by a local oscillator (not shown) installed within the tuner 10. The mixed signals correspond to a frequency difference between the channel-selected signals and the oscillating signals of the local oscillator. The signals under this condition are an intermediate frequency (to be called IF below), and the IF is predetermined in each broadcasting method.
In the case of the NTSC (National Television System Committee) broadcasting method, the IF value for each channel is 45.75 MHz for video signals, and 41.25 MHz for sound signals. The reason why the received signals are convened into the IF signals is for facilitating the demodulation.
The IF signals are output to an IF signal output terminal IF of the tuner 10, and are amplified by an IF signal amplifier 20. Then they are input into a saw-tooth filter 30 by which the signals are deprived of extra channel band signals before being output.
The output signals are demodulated by a demodulator 40. During the demodulation, in order to demodulate the video signals to standard signals, the demodulator 40 is provided with a video signal detecting part 42, which includes an inductor L and a capacitor C in parallel. The signals demodulated by the demodulator 40 are output to a video signal compensating part 50 and a sound signal detecting part 60 respectively. The video signal compensating part 50, which is installed on an output side of the demodulator 40, includes a sound trap 52, a picking part 54 and a buffer 56.
Now the procedure for detecting and outputting the video signals will be described. The signals which are demodulated by the demodulator 40 are input into the sound trap 52 so as to be deprived of sound signals. The sound trap 52 filters off the frequency band containing the sound signals, so that only the video signals should be output. During the removal of the sound signals in the sound trap 52, the video signals of high band level are damped. The damped video signals are compensated by the picking part 54 and, then, are input to the buffer 56 so as to be adjusted to a proper output level. These output level-adjusted video signals are provided to a video signal processing part (not shown).
The sound signal detecting part 60 includes a sound band pass filter 62, a frequency modulation (FM) detector 64 and a de-emphasis circuit 66. This sound signal detecting part 60 will now be described as to its operation. The signals which have been demodulated by the demodulator 40 are input into the sound band pass filter 62, so that extra sound band signals should be filtered off from the demodulated signals. Then the FM detector 64 detects FM signals from the signals which have been filtered by the sound band pass filter 62. An FM oscillating part 70 is connected to one side of the FM detector 64.
The FM oscillating part 70 furnishes a certain frequency which is set by the broadcasting station. For example, in the NTSC method, the furnished oscillating frequency is 4.5 MHz, while in the PAL method, the furnished oscillating frequency is 5.5 MHz or 6.0 MHz.
The sound signals which have been detected by the furnished oscillating frequency are input into the de-emphasis circuit 66. Then the high frequency band output of the sound signals is damped by a time constant of an internal circuit of the de-emphasis circuit 66 and, then, the signals are output to a sound processing part (not shown). The reason why the high frequency band signals are damped is that the broadcasting station carries out a pre-emphasis process to reinforce the high frequency band signals for eliminating the noise which is generated during the transmission. Thus, the output characteristics for the respective frequency band of the sound signals output to the sound signal processing part are made uniform by damping the output of the high frequency band output.
However, in the normal case, the sound carder frequency has to be received with a certain band width depending on the broadcasting method, as described above by way of example based on the picture signal carrier frequency. However, in the actual case, the frequency can be shifted depending on the transmitting state of the broadcasting station and on the propagation medium. That is, if a frequency of a picture signal carrier is shifted, the sound carrier frequency separated by a certain band from the picture signal carder frequency is also shifted, with the result that an accurate detection cannot be carried out, thereby generating noise during the outputting of the sound signals.
In more detail, the conventional receiving device of FIG. 1 is described referring to FIG. 2, which illustrates the frequency band of the signals detected after the demodulation. In FIG. 2, reference code P indicates a picture signal carder frequency, C indicates a color signal carrier frequency, and S indicates a sound signal carrier frequency. As shown in FIG. 2, generally, the difference between the picture signal carrier frequency P and the sound signal carrier frequency S is about 4.5 MHz, and this is the same for all the channels.
However, if the sound signal carrier frequency is shifted by the transmitting state of the broadcasting station or by the propagating medium as described above, the carrier frequency P of the picture signals is transmitted as shown by the dotted lines in FIG. 2. That is, the carrier frequency P of the picture signals is shifted. Further, the carrier frequency S of the sound signals which is separated from the carrier frequency P of the picture signals by about 4.5 MHz is also shifted. This chain reaction is generated whenever the carder frequency of the picture signals has been shifted, since the carrier frequency S of the sound signals is always to be separated by a certain band width from the carder frequency P of the picture signals.
For the reason as described above, when the sound signals are detected and output, noise is generated.
Meanwhile, U.S. Pat. No. 5,177,613 discloses a technique in which the noise is damped by using a single saw-tooth filter for filtering the signals tuned in the conventional manner, thereby providing a high quality wide band sound signal processing circuit. However, this is a device provided with a saw-tooth filter having band characteristics for both audio signals and video signals.